The present invention relates to an exposure control method for controlling exposure of workpieces such as printed-circuit boards.
Printed-circuit boards or other workpieces to be exposed to light for pattern formation are combined with a mask having a predetermined pattern in exposing the workpiece to an image corresponding to the pattern with an exposure device. Since it is general practice to expose a multiplicity of workpieces successively to a single mask pattern, the mask and the workpieces are treated separately from each other. More specifically, a mask is manually attached to a transparent plate in an opened exposure unit in an exposure device, and then a desired number of workpieces are supplied into the exposure apparatus in which they are attached individually and sequantially to the transparent plate. Thereafter, the workpieces are exposed and taken out of the exposure apparatus.
The foregoing prior exposure control process has proven relatively convenient in the case where a multiplicity of workpieces are successively exposed with a single mask. However, exposure of a relatively small number of workpieces, say 20, with a single mask or two masks for both-sided exposure, makes it quite tedious and time-consuming to attach and remove the workpieces to and from the transparent plate.
The conventional exposure device suffers from various other disadvantages. Specifically, since the transparent plate is positioned centrally in the exposure unit in the exposure device, it is troublesome to open the exposure unit to expose the transparent plate out of the exposure device. During the mask attachment process, airborne dust particles tend to be attracted to the exposed transparent plate and the mask attached thereto, resulting in a greater yield of defective products. Various mechanisms in the exposure device are arranged and driven around the central transparent plate therein. Therefore, no sufficient space is available around the transparent plate for easy manual operation to attach and remove the masks to and from the transparent plate. Masks are marked with many symbols indicative of the kinds of the masks. The marked area is contiguous to an outer peripheral grip edge portion of a certain width which is to be gripped for attachment or removal, so that the markings will be prevented from being rendered illegible by dirt from fingers at the time of handling the mask. Such a grip zone reduces a mask area available for the mask pattern.
In an effort to eliminate the above prior art difficulties, there has recently been devised an exposure device with a container disposed adjacent thereto which houses therein turrets accommodating respective masks and turrets each accommodating a desired number of workpieces, the turrets being vertically displaceably arranged as a stack. In operation, a turret carrying a desired mask to be used is vertically moved to a prescribed position, and then moved into the exposure device and stopped in position over the transparent plate. Thereafter, the mask is lifted out of the turret by a transfer mechanism in the exposure device, during which time the turret is retracted into the container, and the transfer mechanism is lowered to position the mask over the transparent plate. Subsequently, a turret with a workpiece accommodated therein is advanced over the transparent plate and the workpiece is mounted in position over the transparent plate. Then, the workpiece is exposed to light. This arrangement can eliminate various shortcomings related to the handling of masks as the masks can be automatically attached and detached with respect to the transparent plate. However, because the container and a computer for controlling movement of the container under a program are necessary, the cost of the overall installation is increased and a space needed for operating the exposure device is large. In addition, a turret with a workpiece carried therein must be moved from the container toward the transparent plate and vice versa each time a workpiece is to be positioned over the transparent plate, a process which results in an increased period of time required for a single workpiece to be exposed. Therefore, the rate of production is relatively poor.